Such a device is described in Applicant's U.K. Pat. No. 1,062,701. This device has walls defining a settling chamber having an inlet opening and an outlet opening in its upper region. Means are provided for connecting the chamber in series with the liquid supply line. A float is located in the chamber and has a specific gravity higher than that of the supply liquid and lower than that of the foreign liquids. Electrical switch means having means for connection to an electrical warning signal are operable by the float so that when the float is in an upper position the circuit is closed and when the float is in a lower position the circuit is open. A warning signal is thus produced when the upper surface of the foreign liquids trapped in the settling chamber reaches a predetermined level.
The switch means includes two spaced conductors within the chamber and electrically insulated from each other and a bridge member, which comprises an annulus of electrically conductive material on the float for bridging the space between the conductors to establish an electrical path between them when the float is in its upper position.
A foraminous element is provided in the settling chamber and arranged in such a way that liquid flowing from the inlet opening to the outlet opening will pass at least once through the foraminous element. The foraminous element is part of a cage within the settling chamber and provides a foraminous wall between the inlet and outlet openings of the chamber.
A drain outlet valve is provided for the lower region of the settling chamber to permit foreign liquids which are separated out and trapped in the settling chamber to be drained from the chamber.